A magnetoresistive element has as a basic structure a structure comprising a reference layer having invariable magnetization, a storage layer having variable magnetization, and a nonmagnetic layer (tunnel barrier layer) between the reference layer and the storage layer. When the magnetization direction of the reference layer is the same as that of the storage layer, the magnetoresistive element enters a low resistance state (parallel state), and this state will be referred to, e.g., a “0”-write state. On the other hand, when the magnetization direction of the reference is opposite to that of the storage layer, the magnetoresistive element enters a high resistance state (anti-parallel state), and this state will be referred to as, e.g., a “1”-write state.
A write operation for making the magnetoresistive element enter the parallel or anti-parallel state is performed by causing, in the case of adopting, e.g., a spin transfer torque (STT) writing, a write current (spin-polarized electrons) which reverses magnetization of the storage layer to flow in the magnetoresistive element. The STT writing is featured in that when the volume of the storage layer is decreased, the write current is also decreased. It is therefore a promising writing as a method which can achieve miniaturization and decreasing of the current at the same time.
However, if the volume of the storage layer is decreased due to the miniaturization, magnetization posterior to writing easily fluctuates due to an agitation of heat, an external magnetic field or the like, as a result of which the stability (retention) of magnetization of the storage layer which is posterior to writing is worsened.